This will restart the clock/timer on the selected voucher and leave the associated MAC addresses registered to it. This option would be used if you wish to extend the timeframe allowed by an active device. These devices would continue to have access until original expiration is reached, at which point they'll be asked to re-enter the voucher code to use the timeframe extension.
This will clear the associated MAC addresses registered to it and restart the clock/timer. This option would be used if you wish to make this previously used voucher into a new voucher, allowing other devices to gain access with a code that's possibly reached its maximum allowed devices or is running out of time. Currently active devices will remain authentication until original expiration is reached.